Synthesis Gas

Synthesis gas ("syngas"), a gaseous mixture comprised mostly of H2 and CO, is produced by the steam reforming or partial oxidation of hydrocarbon feedstocks or a combination of both processes (tandem reforming). Syngas is utilized in the production of an array of chemicals and energy products including but not limited to methanol, ammonia, synthetic fuel (Fischer-Tropsch products), electricity, steel-making, synthetic natural gas (SNG), and hydrogen. The H2/CO ratio required for downstream product generation is adjustable with operational modifications and additional gas shift processes.

In general, gaseous and light oil hydrocarbon are converted into syngas using catalytic steam-reforming or partial oxidation. Heavier oil and solid feedstocks like refinery by-products (residual oil, asphaltene, and petroleum coke) and coal are converted into syngas through gasification reactions in a reducing environment. Generally, the oil/solid based syngas contains unwanted compounds such as hydrogen sulfide, cyanides, and ammonia. Downstream processes are used to remove the unwanted compounds and to adjust the H2/CO ratio to its desired amount. These water-gas-shift reactions, acid gas removal technologies like RECTISOL®or amine-based designs, pressure-swing-adsorption and membrane separation can all be employed as needed.

Steam reforming is the most widely used process for generating hydrogen-rich synthesis gas from hydrocarbon-rich feed materials such as natural gas and naphtha. The feedstocks are converted into syngas using steam generated from process heat or combusted flue gas in catalytic tube reactors. Linde is a leading supplier of steam reformer plants with more than 200 constructed units producing capacities of synthesis gas from 1,000 to over 120,000 Nm³/h.

Linde is one of the leading providers of partial oxidation (POx) plants world-wide, and its process concept covers all type of hydrocarbon feedstocks and resulting products. Linde’s outstanding expertise is backed by more than 30 years of experience and in being the only company worldwide that engineers, installs, and operates POx plants. One of the four POx plants operated by Linde is the world's largest natural gas-fed plant (200,000 Nm³/h H2+CO).

The tandem reformer is an integration of steam reforming and partial oxidation for the generation of synthesis gas to achieve desired hydrogen to carbon monoxide ratios. A hydrocarbon-steam mixture is first reformed in tube reactors and then partially oxidized with oxygen. The escaping synthesis gas heats the tube reactors. The process heat from the partial oxidation is thus used for the steam reforming process.

CO shift conversion

Syngas generally contains hydrogen, carbon monoxide, carbon dioxide, methane, and water plus additional components whose presence is dependent on the feedstock used for syngas generation. To make adjustments to the ratio of hydrogen to carbon monoxide in the syngas to meet downstream process requirements, a catalytic based water-gas shift reaction (CO shift conversion) can be used. In this reaction, carbon monoxide in the syngas is converted to carbon dioxide, and additional hydrogen is generated from the water present in the syngas.

There are three different types of the CO shift conversion:

High-Temperature Shift: reaction taking place around 570 to 840°F, reducing the carbon monoxide in the syngas to approximately 2.5% mol

Medium-Temperature Shift: reaction taking place around 430 to 520°F, reducing the carbon monoxide in the syngas down to approximately 0.5% mol

Low-Temperature Shift: reaction taking place around 350 to 480°F, reducing the carbon monoxide down to approximately 0.2% mol, typically installed downstream of the high-temperature shift to maximize hydrogen production.

RECTISOL® is a physical acid gas removal process using an organic methanol-based solvent at subzero temperatures. RECTISOL® can purify synthesis gas down to 0.1 vppm total sulfur (including H2S and COS) and CO2 into ppm range. Commercial scale RECTISOL® wash units are operated world-wide for the purification of hydrogen, ammonia, methanol, syngas and for the production of high purity carbon monoxide.

The main advantages of this process are low energy consumption, the use of an inexpensive and readily available solvent, and the process configuration flexibility. A special feature of the process is the coil-wound heat exchanger that support energy efficiency and plant economics.

RECTISOL® was independently developed by Linde and Lurgi. The patents and trade mark are used in common.